!676 【6.x】新增CollectorUtil.reduceListMap()用于Collection<Map<K,V>> => Map<K,List<V>>，优化groupingBy中对null的处理

Merge pull request !676 from 阿超/v6-dev

hutool-core/src/main/java/cn/hutool/core/stream/CollectorUtil.java

@@ -2,6 +2,7 @@ package cn.hutool.core.stream;
 
 import cn.hutool.core.lang.Opt;
 import cn.hutool.core.text.StrUtil;
+import cn.hutool.core.util.ArrayUtil;
 
 import java.util.Collections;
 import java.util.EnumSet;
@@ -9,6 +10,8 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
 import java.util.List;
+import java.util.ArrayList;
+import java.util.Objects;
 import java.util.StringJoiner;
 import java.util.function.BiConsumer;
 import java.util.function.BinaryOperator;
@@ -104,7 +107,10 @@ public class CollectorUtil {
 		final BiConsumer<Map<K, A>, T> accumulator = (m, t) -> {
 			final K key = Opt.ofNullable(t).map(classifier).orElse(null);
 			final A container = m.computeIfAbsent(key, k -> downstreamSupplier.get());
+			if (ArrayUtil.isArray(container) || Objects.nonNull(t)) {
+				// 如果是数组类型，不需要判空，场景——分组后需要使用：java.util.stream.Collectors.counting 求null元素个数
 				downstreamAccumulator.accept(container, t);
+			}
 		};
 		final BinaryOperator<Map<K, A>> merger = mapMerger(downstream.combiner());
 		@SuppressWarnings("unchecked") final Supplier<Map<K, A>> mangledFactory = (Supplier<Map<K, A>>) mapFactory;
@@ -211,5 +217,39 @@ public class CollectorUtil {
 		};
 	}
 
+	/**
+	 * 聚合这种数据类型:{@code Collection<Map<K,V>> => Map<K,List<V>>}
+	 * 其中key相同的value，会累加到List中
+	 *
+	 * @param <K> key的类型
+	 * @param <V> value的类型
+	 * @return 聚合后的map
+	 */
+	public static <K, V> Collector<Map<K, V>, ?, Map<K, List<V>>> reduceListMap() {
+		return reduceListMap(HashMap::new);
+	}
+
+	/**
+	 * 聚合这种数据类型:{@code Collection<Map<K,V>> => Map<K,List<V>>}
+	 * 其中key相同的value，会累加到List中
+	 *
+	 * @param mapSupplier 可自定义map的类型如concurrentHashMap等
+	 * @param <K>         key的类型
+	 * @param <V>         value的类型
+	 * @param <R>         返回值的类型
+	 * @return 聚合后的map
+	 */
+	public static <K, V, R extends Map<K, List<V>>> Collector<Map<K, V>, ?, R> reduceListMap(final Supplier<R> mapSupplier) {
+		return Collectors.reducing(mapSupplier.get(), value -> {
+					R result = mapSupplier.get();
+					value.forEach((k, v) -> result.computeIfAbsent(k, i -> new ArrayList<>()).add(v));
+					return result;
+				}, (l, r) -> {
+					r.forEach((k, v) -> l.computeIfAbsent(k, i -> new ArrayList<>()).addAll(v));
+					return l;
+				}
+		);
+	}
+
 
 }

hutool-core/src/test/java/cn/hutool/core/collection/CollStreamUtilTest.java

@@ -153,7 +153,7 @@ public class CollStreamUtilTest {
 		// 对null友好
 		final Map<Long, Map<Long, Student>> termIdClassIdStudentMap = CollStreamUtil.group2Map(Arrays.asList(null, new Student(2, 2, 1, "王五")), Student::getTermId, Student::getClassId);
 		final Map<Long, Map<Long, Student>> termIdClassIdStudentCompareMap = new HashMap<Long, Map<Long, Student>>() {{
-			put(null, MapUtil.of(null, null));
+			put(null, MapUtil.empty());
 			put(2L, MapUtil.of(2L, new Student(2, 2, 1, "王五")));
 		}};
 		Assert.assertEquals(termIdClassIdStudentCompareMap, termIdClassIdStudentMap);
@@ -215,7 +215,7 @@ public class CollStreamUtilTest {
 				new Student(1, 2, 1, "李四"));
 		final Map<Long, List<Student>> termIdStudentsMap = CollStreamUtil.groupBy(students, Student::getTermId, Collectors.toList());
 		final Map<Long, List<Student>> termIdStudentsCompareMap = new HashMap<>();
-		termIdStudentsCompareMap.put(null, Arrays.asList(null, null));
+		termIdStudentsCompareMap.put(null, Collections.emptyList());
 		termIdStudentsCompareMap.put(1L, Arrays.asList(new Student(1L, 1, 1, "张三"), new Student(1L, 2, 1, "李四")));
 		Assert.assertEquals(termIdStudentsCompareMap, termIdStudentsMap);
 

hutool-core/src/test/java/cn/hutool/core/stream/CollectorUtilTest.java

@@ -0,0 +1,37 @@
+package cn.hutool.core.stream;
+
+import cn.hutool.core.map.MapUtil;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * CollectorUtilTest
+ *
+ * @author VampireAchao
+ * @since 2022/7/3
+ */
+public class CollectorUtilTest {
+
+	@Test
+	public void reduceListMapTest() {
+		Set<Map<String, Integer>> nameScoreMapList = StreamUtil.of(
+				// 集合内的第一个map，包含两个key value
+				MapUtil.builder("苏格拉底", 1).put("特拉叙马霍斯", 3).build(),
+				MapUtil.of("苏格拉底", 2),
+				MapUtil.of("特拉叙马霍斯", 1),
+				MapUtil.of("特拉叙马霍斯", 2)
+		).collect(Collectors.toSet());
+		// 执行聚合
+		Map<String, List<Integer>> nameScoresMap = nameScoreMapList.stream().collect(CollectorUtil.reduceListMap());
+
+		Assert.assertEquals(MapUtil.builder("苏格拉底", Arrays.asList(1, 2))
+						.put("特拉叙马霍斯", Arrays.asList(3, 1, 2)).build(),
+				nameScoresMap);
+	}
+}